There is Internet, sites open every other time. Gateway problem?

Question

Computer networks

I ran into a very strange problem. While merging two offices into one single local network, when setting up the equipment, I came across interesting Internet settings from a local provider. The gateway and IP address given to us were in different networks. At the same time, Windows cursed when setting up, but everything worked, while Linux cursed and refused to work with such settings. I called the provider, they were surprised how this could happen, they said wait, we'll figure it out. While I was waiting, I decided to try to score the first addresses from the subnet as a gateway, but to my surprise, the Internet worked on any of the subnet addresses. This is where I completely fell apart. How can this be?!
As a result, they gave me the correct gateway address. Everything worked. A day passed and a new story began. There is Internet, sites are pinged both by ip and by name, teamviewer works - everything seems to be ok, but the pages do not open in the browser. I tried this and that, and nothing works. I decided to forcibly enter the gateway address from the remote office and, lo and behold, it all worked! The jamb means somewhere at the level of the gateway of provider?? Ok, I called back in those support, checked they said everything is ok. Returned everything as it was - it works.
The next morning it all happened again. I immediately break through the remote gateway on the DHCP server, reboot the machines - everything works. Again I called back to those support, but the question hung, how to diagnose the problem in this situation? And what kind of a problem can it be when there are pings, applications work via the Internet, and sites either load or not? I am sure that I will have to butt heads with the provider, we need to stock up on an evidence base :)
The solution is under verification: I read the forums, it seems that there is a problem with the bridge and MTU of the EoIP interface. It is advised to hard manually hammer in MTU 1500 on the interface. I did everything - we will get the result soon, I hope it helps.
UPD solution helped! Hope it helps someone else too.